[#22] uboot needs to initialize charging much sooner in boot process

Details
In the case of a really low battery, the device often doesn't have enough power
to start charging because the boot process doesn't get that far. On boot, uboot
seems to initialize the network before enabling USB charging; this takes ~2-3
seconds, and is usually the time during which a low battery will cause the
device to die. USB charging should be enabled as early as possible in the boot
process to avoid this.

Details
In the case of a really low battery, the device often doesn't have enough power
to start charging because the boot process doesn't get that far. On boot, uboot
seems to initialize the network before enabling USB charging; this takes ~2-3
seconds, and is usually the time during which a low battery will cause the
device to die. USB charging should be enabled as early as possible in the boot
process to avoid this.

Actually, it's not the network (charging is in fact initialized before network)
-- it was the NAND check that's delaying things. Enabling charging should
ideally be moved to the xloader, if possible...